High-speed, high performance watercraft, as used in both military and civilian application, subject the passengers to repetitive high G-forces resulting from the sudden deceleration of the watercraft as it falls off waves or hits waves while going at a high forward speed and a high angle of attack. Such repetitive impacts are both debilitating to the watercraft's occupants, preventing them from carrying out their tasks, and further may result in physical injury.
Shock mitigation is minimizing the effects of a shock when a watercraft, navigating at high speed, hits a wave or a series of waves. As above, these effects can cause fatigue and injuries to the watercraft's passengers and crew especially when subjected to prolonged periods of constant impacts. Whilst a well designed and built watercraft can mostly withstand the shocks caused by these impacts, the passengers and crew experience an uncomfortable ride which reduces physical, cognitive and psychomotor performance and increases the risk of acute and chronic musculoskeletal injuries.
The effect of prolonged body movements and of the forces acting on the musculoskeletal system due to riding in high speed watercraft is, at the very minimum, fatigue. At worst, it can result in serious injury or death. Fatigue due to vibration is caused by prolonged muscle activity, both voluntary and involuntary, resulting from the body's attempt to counteract the vibration. The muscle tissue and organs themselves act as shock absorbers that try to dampen vibration and can become fatigued over time. As fatigue continues, the potential for declining work performance and injury increases due to the unpredictable nature of shocks that come from high speed navigation in significant waves.
U.S. Pat. No. 5,810,125 discloses an active shock-absorbing boat seat system that has a seat system mounted to the boat deck through an active shock absorber. Sensors monitor both the shock to be passed from the deck to the seat as well as the shock actually received by the seat after passing through the shock absorbing system. A controller monitors the shock levels and provides a continuous control signal to the shock-absorbing unit to control the response of the shock-absorbing unit during the duration of the shock. The control system can provide for adjustment of various operating parameters for the system, including initial position of the seating system, overall ride stiffness, maximum allowable shock, and other parameters.
However, a disadvantage of U.S. Pat. No. 5,810,125 is that to mitigate shock, the system requires the use of an array of components such as complex electronic controllers, sensor units, electrohydraulic servo actuators and the like. This increases the costs and potential for failure and may require ongoing maintenance especially in a marine environment.
A purely mechanical arrangement for a shock absorbing mounting system for a high speed watercraft is disclosed in WO 1992/012892. This arrangement includes two parallel rigid arms pivoted at both ends of the arms. A disadvantage of arms that are pivoted at both ends is that they do not flex during compression and therefore do not provide any restorative force. Thus, this configuration requires a separate spring, such as a coil spring interposed between the seat and the base. This spring along with the four pivot points adds weight, cost and complexity to the design. Furthermore, each additional pivot can contribute to additional ‘free-play’ which can produce noise and unwanted movement.
U.S. Pat. No. 5,505,521 discloses a sprung seat frame which comprises a parallel leaf spring arrangement, wherein the leaf springs are fixed or clamped at both ends such that they maintain a constant angle at both ends with out any rotation. During compression, the leaf springs become very stiff and are only able to flex in the middle, adopting an S-shape curvature. The use of computer modelling such as Finite Element Analysis (FEA) software analysing strength (compressive tensile and shear) and flexural modulus/Young's Modulus data has shown that the configuration disclosed in U.S. Pat. No. 5,505,521 is at least four times stiffer compared to a configuration where one end of each spring is allowed to pivot freely. In order to achieve the required flex, springs configured such as those disclosed in U.S. Pat. No. 5,505,521 are required to be manufactured out of a thin material resulting in a spring having to perform under high stress. Therefore, these springs need to be manufactured out of expensive high performance materials such as titanium in order to provide an adequate service life. Also, the clamps at the ends of the spring are under high clamping forces and stress requiring the clamps to be manufactured from heavy bent and welded marine-grade stainless steel plate, thus adding to both the weight and cost of the apparatus.
An alternative embodiment is disclosed in U.S. Pat. No. 5,505,521 that includes an adjustment device that is rotated and then fixed to adjust the curvature of the spring permitting mechanical prestressing of the spring arrangement and thus adjustment of the seat frame according to user weight. Once the spring stiffness/height of the seat is pre-set, the apparatus has the same behaviour and limitations as above since the apparatus is configured with parallel leaf springs with a fixed clamp at both ends such that they maintain a constant angle at both ends without any rotation.
Also, a further problem with all of the mechanical arrangements described above is that they only allow primary shock mitigation in one direction i.e. vertical movement only. Optimally, mitigation apparatus also should factor in lateral stability requirements of occupants where a lateral impact force can have a considerable effect on the body. A lateral impact force can lead to excessive lateral movement of the torso and neck resulting in spinal injuries.
There are a number of other specialist shock mitigation apparatus known in the art with varying configurations manufactured by companies such as Shockwave seats, Ullman Dynamics, Coastshox, X-Craft Suspension Seats and Scot Seats to name a few. However, as above the majority of the shock mitigation apparatus manufactured by these companies only allow for primary shock mitigation i.e. vertical movement only. Also due to their designs requiring construction from marine-grade stainless steel, they are complex, heavy, with many parts requiring labour-intensive manufacturing processes, making them expensive to manufacture.
To overcome the problem of the above, Scot Seats have developed an exemplary shock mitigation seat system which in addition to primary mitigation allows for secondary mitigation in the lateral direction. However, as a result of the rigid construction of the mounting point pivot members, additional componentry referred to as a “shuffle system” is required to effect mitigation in the lateral direction. This results in additional costs for manufacture and a more complex arrangement of componentry to achieve its objective than is necessary. Furthermore, this mitigation seat system does not allow for ease of tuneability to alter and/or control flexure within the spring in three planes of movement (longitudinal surge, vertical heave and lateral sway) and axes of rotation (roll, pitch and yaw) depending on occupant and/or particular application.
It should be appreciated from the above, that there is a need for a shock mitigation apparatus which is capable of counteracting impact motion in three planes of movement and axes of rotation, substantially preventing the resulting forces which are transmitted to a seat occupant through the seat structure, from reaching the seat and its occupant; yet the design allows for the use of lightweight cost effective materials avoiding the need for separate bearings or heavy duty clamps. Furthermore, it would be advantageous if the above can be achieved with a simplified design requiring minimal componentry or to at least provide the public with a useful choice.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinence of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.
For the purpose of this specification the term ‘comprise’ and grammatical variations thereof shall have an inclusive meaning—i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements.
Further aspects and advantages of the process and product will become apparent from the ensuing description that is given by way of example only.